As an example of renewable power sources, there is photovoltaic power generation, which is not highly popularized yet. To further spread the use of photovoltaic power generation needs reduction in power generation cost, and this reduction requires increase in the efficiency and lowering of cost for solar cells. The known solar cells which are actually used now are ones of monocrystalline Si, polycrystalline Si, amorphous Si, chalcopyrite compounds such as CIGS (copper indium gallium selenide), and CdTe. While these solar cells have been reduced in cost, the efficiency cannot be increased enough because of the single junction structure thereof and the highest possible efficiency achieved so far is in the low 20-percent range.
As a solar cell that can have higher efficiency, a multijunction solar cell is being proposed. Specifically, for a triple junction solar cell with a laminated structure of 3 thin films respectively having 3 types of group III-V compounds (InGaP/GaAs/InGaAs), a high efficiency of 37.7%, which exceeds the theoretical limit for single junction solar cells, is achieved. The group III-V compound thin films, however, need to be formed by epitaxial growth, which requires an expensive manufacturing device for MOCVD (metal organic chemical vapor deposition), for example. Accordingly, the watt unit price (manufacturing cost divided by power generation amount) as a parameter of solar cell manufacturing cost is two to three orders of magnitude higher for the triple junction solar cells of the group III-V compound thin films than that for single junction solar cells. The power generation cost, which the applicants discuss herein as a key issue, is therefore orders of magnitude higher for the triple junction solar cells than that for single junction solar cells.
As a candidate of future solar cells which can achieve higher efficiency and lower cost at one time, concentrating solar cells are being proposed. The advantage of the concentrating solar cells is that the area of a panel can be smaller because of high efficiency of light collecting. The disadvantages are that a light collector has a complicated, heavy, and large structure if a large-size lens or reflecting mirror is used for collecting light and a sunlight tracking device is additionally necessary. The disadvantages lead to extremely high cost of power generation, contrary to our expectation.